4 W. V. R. WBLDON. 



fig. 4 belongs, shows the condition of the mesoblast shortly 

 after its origin. 



The blastopore being funnel shaped, with its narrow opening 

 directed downwards, it appears in a lateral longitudinal section 

 as a pit, closed below, and from its closed extremity the meso- 

 blast grows forwards as a solid cap, separate from epiblast and 

 hypoblast. 



Transverse sections show that the mesoblast is in connection 

 not only with the walls of the blastopore, but also with the 

 axial strip of invaginated hypoblast. Figs. 6 — 13 are selected 

 from a series of transverse sections of an embryo slightly older 

 than that represented in fig. 4, and show the relations of the 

 mesoblast. The figures are arranged in order from behind 

 forwards, fig. 6 being posterior. Figs. 6 — 9 pass through the 

 blastopore, and a sheet of mesoblast, continuous with its walls, 

 is seen growing out of each side. In figs. 10 and 11, which 

 pass through the posterior embryonic region in front of the 

 blastopore, each sheet of mesoblast is seen to be free laterally, 

 but to be continuous near the middle line with the axial strip 

 of hypoblast, the cells of which will give rise to the notochord, 

 and are easily distinguishable from the more peripheral hypo- 

 blast cells by their more elongated forms and by being more 

 than one layer deep. 



This mode of origin of the mesoblast, however, only holds 

 good for the posterior part of the embryo. Anteriorly (fig. 11) 

 the mesoblastic sheet loses its connection with the axial hypo- 

 blast and finally disappears (fig. 12), being replaced by 

 branched cells, which are budded ofi^, partly from the axial, 

 partly from the lateral hypoblast. This mode of origin of the 

 anterior mesoblast has been overlooked by Hofi'mann. 



The account above given is obviously in complete accord 

 with the observations of Balfour,^ who described a stage a 

 little later than that represented in figs. 6 — 13, with a widely- 

 open, neuro-enteric canal, and a sheet of mesoblast on each 



' Balfour, " On the Early Development of the Lacertilia," &c., ' Quart. 

 Journ. Mic. Sci.,' xix. 



